Photographers often use a flash or strobe light to achieve desired effects in their photographs. Many modem cameras include a built-in electronic flash lamp so that the use of flash is convenient. Sometimes, the flash is used to provide the primary illumination of a scene when insufficient light is available from other sources. The light that is available from other sources is often called ambient light.
In some cases, there may be sufficient ambient light for exposing a photograph without the use of a flash, but the distribution of the ambient light is such that the resulting photograph is disappointing. This is often the case with brightly backlit subjects such as a person in a relatively dark room with a sunlit window in the background. The camera may adjust its exposure settings to the average brightness of the scene, thereby severely underexposing the person.
A technique for improving photographs in this situation is the use of fill flash. Fill flash is the use of a flash or strobe light, usually built into or mounted on the camera, to add light to a scene that may have enough ambient light for a photograph, but which can be improved by the use of the flash. In the above example, the camera flash can boost the illumination of the person but will have a negligible effect on the brightness of the sunlit objects outside the window. In this way, the relative illumination of the person and the outdoor objects are brought more closely together, allowing for better relative exposure of the various parts of the scene.
Fill flash may be used for other purposes as well. Fill flash can add highlight emphasis to foreground subjects. It can also be used to soften harsh shadows that may be present, for example when people being photographed are lit by strong near-overhead sunlight.
While fill flash can improve the quality of photographs in some situations, it has several disadvantages. In some situations, such as in museums, flash photography may not be allowed. In other circumstances, such as at a wedding or formal dinner, flash photography may be considered impolite.
Fill flash may also create some undesirable shadow effects. For example, when a person is photographed using flash, one side of the person may be limned in the photograph by a harsh and objectionable shadow falling on objects just behind the person.
Electronic flash also consumes significant electrical energy. Using the camera flash may rapidly deplete the batteries powering the camera, thereby limiting the useful life of the camera between battery replacements or rechargings.
Electronic flash can also produce an undesirable effect called “redeye” when the light from the flash is reflected back to the camera from the retina of the subject's eye. Methods of reducing the “redeye” effect such as using a preflash to constrict the iris of the subject add undesirable delay in the shutter response of the camera.
And finally, with some cameras the user must decide before taking a photograph whether to use fill flash or not. Heretofore, the effect of flash could not be easily added or removed after the photograph is taken. While some cameras can automatically decide whether to use fill flash, photographs taken by an automated camera are still subject to all of the other disadvantages of fill flash. Furthermore, the photographer may disagree with the camera's decision and wish for a way to modify the result.
What is needed is a camera that can add or adjust the effects of fill flash after a photograph is taken, can provide the beneficial effects of fill flash while avoiding the undesirable effects, and can gain the benefits of fill flash in situations where flash photography may not be used.